Editing Aztec Code (section)

=== Mode message ===
The mode message encodes the number of layers (''L'' layers encoded as the integer ''L''−1), and the number of data codewords (''D'' codewords, encoded as the integer ''D''−1) in the message.  All remaining codewords are used as check codewords.

For compact Aztec codes, the number of layers is encoded as a 2-bit value, and the number of data codewords as a 6-bit value, resulting in an 8-bit mode word.  For full Aztec codes, the number of layers is encoded in 5 bits, and the number of data codewords is encoded in 11 bits, making a 16-bit mode word.

The mode word is broken into two or four 4-bit codewords in [[Galois field|GF(16)]], and 5 or 6 Reed–Solomon check words are appended, making a 28- or 40-bit mode message, which is wrapped in a 1-pixel layer around the core. Thus a (15,10) or (15,9) Reed-Solomon code (shortened to (7,2) or (10,4) respectively), over GF(16) is used.

Because an L+1-layer compact Aztec code can hold more data than an L-layer full code, full codes with less than 4 layers are rarely used.

Most importantly, the number of layers determines the size of the Reed–Solomon codewords used.  This varies from 6 to 12 bits:
{|class="wikitable" style="text-align:center;"
|+ Aztec code finite field polynomials
! Bits || Field || Primitive polynomial || Generator polynomial (decimal coefficients) || Used for
|-
| 4 || GF(16) || ''x''<sup>4</sup>+''x''+1 || ''x''<sup>5</sup>+11''x''<sup>4</sup>+4''x''<sup>3</sup>+6''x''<sup>2</sup>+2''x''+1 (Compact code) <br />''x''<sup>6</sup>+7''x''<sup>5</sup>+9''x''<sup>4</sup>+3''x''<sup>3</sup>+12''x''<sup>2</sup>+10''x''+12 (Full code)|| Mode message
|-
| 6 || GF(64) || ''x''<sup>6</sup>+''x''+1 ||depends on number of error correction words|| 1–2 layers
|-
| 8 || GF(256) || ''x''<sup>8</sup>+''x''<sup>5</sup>+''x''<sup>3</sup>+''x''<sup>2</sup>+1 ||depends on number of error correction words|| 3–8 layers
|-
| 10 || GF(1024) || ''x''<sup>10</sup>+''x''<sup>3</sup>+1 ||depends on number of error correction words|| 9–22 layers
|-
| 12 || GF(4096) || ''x''<sup>12</sup>+''x''<sup>6</sup>+''x''<sup>5</sup>+''x''<sup>3</sup>+1 ||depends on number of error correction words || 23–32 layers
|}
The codeword size ''b'' is the smallest even number which ensures that the total number of codewords in the symbol is less than the limit of 2<sup>''b''</sup>−1 which can be corrected by a Reed–Solomon code.

As mentioned above, it is recommended that at least 23% of the available codewords, plus 3, are reserved for correction, and a symbol size is chosen such that the message will fit into the available space.